Re: [LTP] Segmentation fault when running sched_setaffinity01 in RHEL5.10GA

[Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>]

Hi,

On 07/25/2014 02:39 PM, Stanislav Kholmanskikh wrote:
> 
> 
> On 07/25/2014 09:38 AM, Xiaoguang Wang wrote:
>> Hi,
>>
>> When we run sched_setaffinity01 in RHEL5.10GA, it occurs a segmentation fault.
>> Below is the possible reason.
> 
> Hi!
> 
> I though I'd checked this test case on OL5 before submission. Let me check it again.

OL5 is based RHEL5?
If it is, please also check your CONFIG_NR_CPUS in kernel config file. If it's 1024 or greater,
this case will not fail. In my environment, this value is 255, thanks!

Regards,
Xiaoguang Wang
> 
>>
>> Hi Jan, would you please help to confirm this problem. I'm afraid RHEL5.10GA is an old
>> distribution, which many people may not use it now, thanks!
>>
>>
>> Glibc provides a encapsulation for the raw kernel sched_setaffinity(2) system call,
>> the corresponding code is below(The version of glibc I used is glibc-2.5-20061008T1257-RHEL5.11Beta):
>> I delete some code just for simple.
>>
>> #######################################################################################
>>
>> /* Size definition for CPU sets.  */
>> # define __CPU_SETSIZE  1024
>> # define __NCPUBITS     (8 * sizeof (__cpu_mask))
>>
>> /* Type for array elements in 'cpu_set'.  */
>> typedef unsigned long int __cpu_mask;
>>
>> /* Basic access functions.  */
>> # define __CPUELT(cpu)  ((cpu) / __NCPUBITS)
>>
>> /* Data structure to describe CPU mask.  */
>> typedef struct
>> {
>>    __cpu_mask __bits[__CPU_SETSIZE / __NCPUBITS];
>> } cpu_set_t;
>>
>>
>> int __sched_setaffinity_new (pid_t pid, size_t cpusetsize, const cpu_set_t *cpuset)
>> {
>>    if (__builtin_expect (__kernel_cpumask_size == 0, 0))
>>      {
>>        int res;
>>
>>        while (res = INTERNAL_SYSCALL (sched_getaffinity, err, 3, getpid (),
>>                                       psize, p),
>>               INTERNAL_SYSCALL_ERROR_P (res, err)
>>               && INTERNAL_SYSCALL_ERRNO (res, err) == EINVAL)
>>           ....
>>
>>        __kernel_cpumask_size = res;
>>      }
>>
>>    /* We now know the size of the kernel cpumask_t.  Make sure the user
>>       does not request to set a bit beyond that.  */
>>    for (size_t cnt = __kernel_cpumask_size; cnt < cpusetsize; ++cnt)
>>      if (((char *) cpuset)[cnt] != '\0')
>>        {
>>          /* Found a nonzero byte.  This means the user request cannot be
>>             fulfilled.  */
>>          __set_errno (EINVAL);
>>          return -1;
>>        }
>>
>>    return INLINE_SYSCALL (sched_setaffinity, 3, pid, cpusetsize, cpuset);
>> }
>> #######################################################################################
>>
>> Glibc in RHEL5.10GA does not provide CPU_ALLOC_SIZE marco, so in ltp testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity.h,
>> we define one.
>> #######################################################################################
>> #ifndef CPU_ALLOC_SIZE
>> #define CPU_ALLOC_SIZE(size) sizeof(cpu_set_t)
>> #endif
>> #######################################################################################
>>
>> Then CPU_ALLOC_SIZE would always return 128 in RHEL5.10GA, that is when we test EFAULT for sched_setaffinity(2),
>> the passed cpusetsize is 128. But look at __sched_setaffinity_new() above, it first call
>> raw sched_getaffinity(2) to get  the size of the kernel cpumask_t, In RHEL5.10GA,
>> this value depends on CONFIG_NR_CPUS, if CONFIG_NR_CPUS is 255, the raw sched_getaffinity(2) will return 32.
>> In this case, __kernel_cpumask_size would be 32, cpusetsize is 128. Give that we're testing
>> EFAULT, cpuset is a invalid pointer, if cnt > 32, it will generate segmentation fault in glibc code,
>> so this case exits abnormally
>>
>> As why this test case can run normally in RHEL6.5GA or RHEL7.0GA, it's because
>> sched_getaffinity(2) in old kernel(RHEL5.10GA) return sizeof(cpumask_t), which totally depends on CONFIG_NR_CPUS.
>> In newer kernel, sched_getaffinity(2) returns the smaller one between min_t(size_t, len, cpumask_size()),
>> here len is the value passed to sched_getaffinity as cpusetsize, cpumask_size() is the max allowed length.
>> so we can ensure __kernel_cpumask_size will never smaller cpusetsize, so the segmentation fault won't occur.
>>
>> So I also think CPU_ALLOC and CPU_ALLOC_SIZE is wrong in testcases/kernel/syscalls/sched_setaffinity/sched_setaffinity.h. We should
>> refer to the implementation in glibc. or we define CPU_ALLOC_SIZE using raw sched_getaffinity as a workaround in older kernel . See below code:
>> #########################################################################################
>>
>>          int ret;
>>          cpu_set_t cst;
>>
>>          memset(&cst, 0, sizeof(cst));
>>
>>          ret = syscall(__NR_sched_getaffinity, getpid(),
>>                        sizeof(cpu_set_t), &cst);
>>          if (ret < 0) {
>>                  fprintf(stderr, "sched_getaffinity failed: %s\n",
>>                          strerror(errno));
>>                  return 1;
>>          } else {
>>                  printf("length of bit mask the kernel uses to represent the CPU"
>>                         ": %d\n", ret);
>>          }
>> #########################################################################################
>>
>> Regards,
>> Xiaoguang Wang
>>
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> .
> 


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